Hormonal changes over the course of pregnancy affect the expression of care by the mother towards her newborn. The precise involvement of the pituitary protein prolactin in the hormonal regulation of maternal behavior is examined in a series of experiments in order to delineate the conditions and sites of prolactin's stimulatory actions on maternal behavior. Experiments are designed to test the hypothesis that during pregnancy the exposure of the brain to prolactin and prolactin-like (placental lactogen) molecules in combination with steroid hormones, progesterone and estradiol, facilitates the rapid onset of maternal behavior at parturition. In the initial study the effects of prolonged and acute prolactin priming on the onset of maternal behavior in steroid-treated hypophysectomized rats are measured. The interactions of prolactin with estradiol and progesterone are then examined to define the steroidal conditions required for prolactin's behavioral action. The specificity of prolactin's action is evaluated by measuring the behavioral effects of other pituitary hormones (growth hormone, LH) and prolactin-like hormones produced by the placenta (placental lactogen). In another study, the effects of treatment with prolactin antagonists on plasma prolactin levels (radioimmunoassay) and the induction of maternal behavior are determined to see whether the facilitation of maternal behavior induced by prolactin can be prevented with prolactin antagonists. Next, possible central sites of prolactin's action are examined by infusing prolactin into the lateral ventricles and implanting prolactin into central loci involved in the regulation of maternal behavior, i.e. medial preoptic area, median raphe nuclei. In a final experiment, the effects of central prolactin administration on the intensity of ongoing (maintenance) maternal behavior are measured. Techniques employed include radioimmunoassays of hormones, stereotaxic surgery, general animal surgery, microinfusion of hormones into the brain, and behavioral testing. These studies will allow us to define the conditions and sites of prolactin's involvement in the induction of maternal care, to determine whether other prolactin-like molecules may affect maternal care, and in a more general sense to advance our understanding of the endocrine and neural events that regulate mother-young interactions in mammals.